Final answer:
Lamarck's key mistake in evolutionary theory was the idea of inheritance of acquired characteristics, which modern genetics disproves. However, his work laid a foundational step in understanding evolution and recognized environmental interaction's role; aspects of his ideas persist in contemporary epigenetics.
Step-by-step explanation:
The core issue with Jean-Baptiste Lamarck's ideas about evolution pertains to the inheritance of acquired characteristics. Lamarck proposed that traits developed during an organism's life in response to its environment could be passed on to its offspring. For example, he suggested a giraffe might extend its neck to reach higher leaves and then pass this longer neck onto its progeny. However, modern genetics has shown that physical or behavioral changes acquired during an organism's life generally do not alter the genetic material passed to the next generation.
While this aspect of Lamarck's theory was mistaken, it's important to acknowledge his contributions to evolutionary biology. He was among the first to suggest a mechanism for how species could change over time, recognizing the importance of the interactions between organisms and their environments. This was a groundbreaking step in the development of evolutionary thought. Notably, scientists today are exploring epigenetics, which shows that environmental factors can affect how genes are expressed, indicating that Lamarck was not completely wrong about the influence of the environment on heredity.